The present invention relates to a semiconductor device formed as an integrated circuit. In particular, the present invention relates to a technique of a long distance transmission circuit between circuit blocks provided on a semiconductor substrate.
On-chip wiring formed on the semiconductor substrate can be represented by a distributed constant line formed of wiring resistance Ru and wiring capacitance Cu as shown in FIG. 9. Especially in recent years, the wiring resistance Ru is increased remarkably by finer wiring in the on-chip long distance transmission on the semiconductor substrate. As a result, bluntness of the received waveform caused by the wiring resistance Ru and wiring capacitance Cu becomes large, resulting in a great obstacle to fast transmission.
As for schemes for transmitting signals over a long distance transmission line which are not restricted to the top of the semiconductor substrate, two transmitter-receiver circuit schemes are basically known. One of them is the voltage transmission scheme shown in FIG. 2A in which voltage signals are transmitted and received in a transmission system having an open receiving end, and it is s scheme used most frequently in transmission on a semiconductor substrate. The other of them is a current transmission scheme shown in FIG. 2B in which current signals are transmitted and received in a transmission system having a terminated receiving end. Results obtained by applying the two schemes to long distance transmission using fine wiring on the semiconductor substrate and comparing rise time values of received waveforms are shown in FIG. 2C. In FIG. 2C, it is supposed that the wiring pitch is approximately 0.28 μm and the wiring resistance Ru and the wiring capacitance Cu per unit length are 510 Ω/mm and 0.25 pF/mm, respectively. As appreciated from FIG. 2C as well, the current transmission scheme is approximately 2.8 times faster than the voltage transmission scheme. The current transmission scheme brings about an effect obtained by making the output impedance of the transmitter circuit and the terminal impedance at the receiving end smaller than the wiring resistance Rt. This effect is brought about by a phenomenon called in general Thomson's arrival current phenomenon. As for a current transmission scheme for conducting transmission by using wiring formed on a semiconductor substrate, a circuit intended to detect and amplify data stored in a memory cell array in an SRAM and described in “Current-Mode Techniques for High-Speed VLSI circuits with application to Current Sense Amplifier for CMOS SRAM's,” IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 26, NO. 4, PP. 525-536, April 1991. is widely used. In this circuit, however, a memory cell corresponding to a transmission circuit is driven by a constant current source, and consequently its output impedance becomes much higher than the wiring resistance, the effect of the above-described phenomenon being not obtained. On the other hand, as for a known technique of a conventional transmission circuit capable of utilizing the effect of the above-described phenomenon, JP-A-7-147092 and JP-A-8-162942 can be mentioned.
FIG. 7 shows a current transmission circuit formed of bipolar transistors and described in JP-A-7-147092. In this current transmission circuit, Q3 in a transmitter circuit 101 and Q5 in the receiver circuit 102 constitute a current switch circuit, and Q4 in a transmitter circuit 101 and Q6 in the receiver circuit 102 constitute another current switch circuit. For example, when an input terminal INp in the transmitter circuit 101 is at its high level and an input terminal INn in the transmitter circuit 101 is at its low level, a base potential of Q3 becomes its high level and a base potential of Q4 becomes its low level. If the base potential of Q3 becomes higher than the base potential VB of Q5, a current which has flown through a resistor in the transmitter circuit 101 and a constant current source I2 until then begins to flow through Q3. If the base potential of Q3 becomes higher than the base potential VB of Q5 by a voltage drop ΔVR caused by wiring 103, all of the current flows through Q3. A relation between Q4 and Q6 is opposite to the relation between Q3 and Q5. Therefore, all of a current flowing through a resistor RL2 and a constant current source I3 which has flown through Q4 until then flows through Q6. Since Q5 turns off, an output terminal Op in the receiver circuit 102 goes high. Since Q6 turns on, an output terminal On goes low. As a result, a voltage signal is output to the output terminals. In this circuit scheme, current signal transmission is implemented by exchanging a current flowing through the resistor RL1 and the constant current source I2 or through the resistor RL2 and the constant current source I3 between the transmitter circuit and the receiver circuit as a current signal. Since a current is always let flow through Q3 and Q4 in the transmitter circuit 101 and Q5 and Q6 in the receiver circuit 102, impedance seen from each of emitters of these transistors becomes very small. As a result, both output impedance of the transmitter circuit and the input impedance of the receiver circuit can be made smaller than the wiring resistance. Therefore, the speed increase effect owing to the Thomson's arrival current effect is obtained. Even if bipolar transistors in the circuit shown in FIG. 7 are replaced by NMOS transistors, similar transmission is possible.
FIG. 8 shows a current transmission circuit formed of MOS transistors and described in JP-A-8-162942. In this current transmission circuit, a current generated by a constant current source I1 in the transmitter circuit 101 flows through wiring 103 or 104 according to potentials at input terminals INp and INn. For example, when the input terminal INp in the transmitter circuit 101 is at its high level and the input terminal INn is at its low level, Q1 turns off and Q2 turns on, and consequently output terminals Dp and Dn in the transmitter circuit 101 becomes the high level and low level, respectively. Since Q2 turns on, a current Il of the constant current source I1 is drawn from an input terminal of the receiver circuit 102 via the wiring 104 at this time. On the other hand, since Q1 turns off, a current Ih obtained by dividing a potential difference between an input terminal Rp in the receiver circuit 102 and a power supply VDD by a sum of load resistance R1 and wiring resistance Rt of the wiring 103 flows into the input terminal Rp in the receiver circuit 102. As a result, a potential difference caused between load means L3 and L4 by Il and Ih is output between output terminals On and Op in the receiver circuit 102 as a voltage signal. In this circuit scheme, current signal transmission is implemented by exchanging the current Ih which flows out when the output of the transmitter circuit is at its high level and the current Il of the constant current source I1 between the transmitter circuit and the receiver circuit as current signals. In this circuit as well, the speed increase effect owing to the Thomson's arrival current effect can be obtained by setting resistances R1 and R2 in the transmitter circuit 101 smaller than the wiring resistance Rt and always letting currents flow through Q11 and Q12 and thereby making the input impedance of the receiver circuit lower than the wiring resistance.